On the fsfs-format7 branch: extend the representation header
data structure to contain the offset between representation start
and actual begin of the plain or deltified data (= header size).

* subversion/libsvn_fs_fs/low_level.h
  (svn_fs_fs__rep_header_t): add header_size member

* subversion/libsvn_fs_fs/low_level.c
  (svn_fs_fs__read_rep_header): set the new member as well


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1442518 13f79535-47bb-0310-9956-ffa450edef68